The present invention relates generally to signal quality estimation in a mobile communication network and, more particularly, to signal quality estimation for the uplink in LTE systems.
Long Term Evolution (LTE) systems use single-carrier frequency-division multiple-access (SC-FDMA) for uplink transmissions. The use of single carrier modulation for the uplink is motivated by the lower peak-to-average ratio of the transmitted signal compared to conventional OFDM, which results in higher average transmit power and increased power amplifier efficiency. The use of SC-FDMA in the uplink, however, gives rise to an inter-symbol interference (ISI) in dispersive channels. It is important to mitigate the effects of ISI so that SC-FDMA can improve power amplifier efficiency without sacrificing performance.
Linear minimum mean square error (LMMSE) receivers in the base station (also known as an eNodeB) can suppress ISI using linear frequency domain equalization. LMMSE receivers are designed to maximize the signal-to-interference-plus-noise ratio (SINR) for each subcarrier component. Though LMMSE improves performance significantly beyond a simple match filtering receiver, further improvements in performance could be obtained with advanced receivers using techniques such as Turbo Soft Interference Cancellation (TuboSIC), or near maximum-likelihood detectors, such as a reduced state sequence estimator (RSSE), Serial Localization with Indecision (SLI) and Assisted Maximum Likelihood Detector (AMLD). These advance receivers are expected to achieve performance very close to the performance of a maximum-likelihood detector.
One problem encountered with the deployment of advanced receivers is obtaining reliable channel quality indication (CQI) estimation and modulation and coding scheme (MCS) selection. CQI estimates are used, for example, for link adaptation and scheduling in the uplink of LTE. Currently, there is no solution for CQI estimation for RSSE, SLI, MLD, or other ML, or near ML, receiver in the uplink in LTE systems.